A cathode active material for a lithium secondary cell used in a cellular phone is disclosed. The cathode active material for the lithium secondary cell and the method the same having a high capacity and a long lifetime, different from LiCoO2 and LiMn2O4, Li(Ni, Co)O2, and V-system oxide that has been researched as the active material for substituting LiCoO2 are provided. The cathode active material for the lithium secondary cell in the next formula 1 is obtained by heating or chemically treating diadochite [Fe2(PO4)(SO4)(OH).6H2O] that is the mineral containing PO43−, SO42−, and OH−. LiaFebMc(PO4)x(SO4)y(OH)z   (1) In the formula, M is at least one element selected from a radical consisting of Mg, Ti, Cr, Mn, Co, Ni, Cu, Zn, Al, and Si, with 0≦a, c≦0.5, 1≦b≦2, 0.5≦x, y, z≦1.5.